Matt Mullenweg responds to Daniel Jalkut’s argument “that the GPL does more to harm collaborative development than it does to help it”:

I’ve never encountered a serious client who chose not to use
WordPress because it was GPL-licensed, and I think it’s hard to
argue that WordPress’s license has had a dampening effect on its
adoption, given its success over competitors with widely varying
licenses.

I think we have an incredibly strong third-party extension,
plugin, and theme community that has flourished, not in spite of the
GPL license, but because of it.

I’ve seen the absence of GPL in practice; there have been times
in the WordPress world when parts of the community have “gone dark”
and claimed their code was under more restrictive licenses, like
used to be common with themes. Every time this cycle starts it
basically kills innovation in that part of the WordPress world until
people start opening up their code again or until a GPL equivalent
is available. I’ve seen this firsthand several times now.

I can’t speak for Jalkut, but none of these three points from Mullenweg address Jalkut’s argument.

Jalkut wasn’t arguing about whether users will not use GPL software; his argument was about developers.

Jalkut never argued that WordPress wasn’t popular or didn’t have a strong extension/plugin/theme community. Jalkut’s argument was that WordPress might have an even stronger extension/plugin/theme community if it were licensed under a BSD-style license.

Jalkut wasn’t arguing in favor of more restrictive licenses; he was arguing in favor of less restrictive ones: BSD/MIT/Apache style ones.

In some sense, Jalkut’s essay could be considered a big “Duh” — a statement of the obvious. To wit: that GPL-licensed software projects discourage participation from developers working on anything other than other GPL-licensed software projects. That’s pretty much the stated goal of the FSF. BSD-licensed projects encourage participation from developers working on just about anything.